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Future Bio Tec

Guidelines for the design and application of particle precipitators for residential biomass combustion

Ingwald Obernberger, Graz University of Technology, Austria

Friedrich Biedermann, Thomas Brunner, Bioenergy 2020+ GmbH, Austria

Olli Sippula, Jorma Jokiniemi, University of Eastern Finland

Christoffer Boman, Umeå University, Sweden

Fredrik Niklasson, Linda Bäfver, SP Technical Research Institute of Sweden

Hans Hartmann, Technology and Support Centre of Renewable Raw Materials, Germany

John Finnan, John Carroll, Crops Research Centre, Ireland

Report within the scope of the ERA-NET Bioenergy Project “FutureBioTec”

October 2012

Disclaimer

The concepts and design guidelines presented are a result of a scientific project. The

implementation and utilisation of the research results is the decision of each individual

person/company. The authors undertake no liability for the utilisation and implementation of

the research work and research results as well as for consequences of the resulting

technology development or plant operation.

Contact:

BIOENERGY 2020+ GmbH

Inffeldgasse 21b, A-8010 Graz, Austria

Email: [email protected]

Tel.: +43 316 873 9201

www.bioenergy2020.eu

ERA-NET Bioenergy Project FutureBioTec Guidelines for the design and application of particle precipitators for residential biomass combustion

Preface

ERA-NET Bioenergy is a network of national research and development programmes focusing on

bioenergy which includes 14 funding organisations from 10 European countries: Austria, Denmark,

Finland, France, Germany, Ireland, The Netherlands, Poland, Sweden and the United Kingdom. Its

mission is to enhance the quality and cost-effectiveness of European bioenergy research

programmes, through coordination and cooperation between EU Member States. The project

FutureBioTec (Future Low Emission Biomass Combustion Systems) has been supported in the period

between October 2009 and September 2012 by ERA-NET Bioenergy under the joint call on Clean

Biomass Combustion from 2009.

The European Union and its member States aim at an increased use of renewable energy in order to

avoid a further increase in atmospheric CO2 concentrations and therefore, the European Commission

actively supports the utilisation of biomass for energy production. However, this aim must be achieved

without increasing other harmful emissions such as fine particulate matter (PM2.5), nitric oxides (NOx),

carbon monoxide (CO) and organic compounds (OGC, PAH). Therefore, especially regarding the

small and medium-scale heating sector, where a great potential for biomass utilisation all over Europe

exists, the promotion of energy from biomass must be accompanied by further technology

development towards low emission combustion systems.

Against this background, the project FutureBioTec aimed to provide a substantial contribution

concerning the development of future low emission stoves and automated small and medium-scale

biomass combustion systems (<20 MWth). Considering the different states of development of the

combustion technologies and capacity ranges addressed, the project focused on the following main

objectives.

• The further development of wood stoves towards significantly decreased CO, OGC, PM and

NOx emissions by primary measures (air staging and air distribution, grate design and

implementation of automated process control systems).

• The improvement of automated furnaces in the residential and the small to medium-scale

(<20 MWth) capacity range towards lower PM and NOx emissions by primary measures

(staged combustion, utilisation of additives as well as fuel blending).

• The evaluation, development and optimisation of secondary measures for PM emission

reduction in residential biomass combustion systems.

In order to reach these objectives, a consortium of 8 research organisations and 2 industrial partners

from 7 European countries collaborated within FutureBioTec (see next page).

This document summarizes experiences from comprehensive test runs of existing particle precipitation

devices for residential biomass combustion systems. The target groups of this report are

manufacturers of residential combustion devices, retailers, and central authorities. Furthermore, the

report may also be of interest to consumers and researchers.

Ingwald Obernberger

Project coordinator

ERA-NET Bioenergy Project FutureBioTecii Guidelines for the design and application of particle precipitators for residential biomass combustion

FutureBioTec project partners

Project coordinator

BIOENERGY 2020+ GmbH (BE2020) in cooperation with Graz University of Technology Institute for Process and Particle Engineering Graz, Austria

Project partners (R&D)

University of Eastern Finland (UEF) Department of Environmental Sciences Fine Particle and Aerosol Technology Laboratory Kuopio, Finland

Technology and Support Centre of Renewable Raw Materials (TFZ) Straubing, Germany

Umeå University (UmU) Energy Technology and Thermal Process Chemistry Umeå, Sweden

Luleå University of Technology (LTU) Division of Energy Engineering Luleå, Sweden

SP Technical Research Institute of Sweden (SP) Division of Energy Technology Borås, Sweden

Institute of Power Engineering (IEn) Thermal Division Department Warsaw, Poland

Teagasc, Crops Research Centre Carlow, Ireland

Industrial partners

Warma-Uunit Ltd, Finland

Applied Plasma Physics AS, Norway

ERA-NET Bioenergy Project FutureBioTec

Guidelines for the design and application of particle precipitators for residential biomass combustion i

Table of contents

1 Introduction and objectives ............................................................................................ 1

1.1 Target group .............................................................................................................. 1

2 Definitions and limitations .............................................................................................. 1

2.1 Basic definitions......................................................................................................... 1

2.2 Limitations ................................................................................................................. 2

3 Design criteria................................................................................................................ 2

3.1 Technology ................................................................................................................ 2

3.2 Efficiency ................................................................................................................... 3

3.3 Installation ................................................................................................................. 4

3.4 Pressure drop ............................................................................................................ 4

3.5 High voltage and electrode ........................................................................................ 4

3.6 Control system........................................................................................................... 5

3.7 Maintenance and availability ...................................................................................... 5

3.8 Applicability................................................................................................................ 6

3.9 Cleaning .................................................................................................................... 6

3.9.1 Solid waste: ash, coke and soot .......................................................................................... 6

3.9.2 Waste water......................................................................................................................... 6

3.10 Safety ........................................................................................................................ 6

3.11 Noise ......................................................................................................................... 7

3.12 Weather conditions .................................................................................................... 7

3.13 Costs ......................................................................................................................... 7

4 Quality assurance.......................................................................................................... 8

4.1 Testing methods ........................................................................................................ 8

5 Conclusions................................................................................................................... 9

6 Acknowledgement ........................................................................................................10

7 Related literature ..........................................................................................................10

ERA-NET Bioenergy Project FutureBioTec Guidelines for the design and application of particle precipitators for residential biomass combustion 1

1 Introduction and objectives

Residential biomass combustion is one of the main sources of PM2.5 in ambient air, besides

traffic and industry. High concentrations of PM2.5 in the air are a health risk and

consequently are regulated in an EU directive [ 1]. Ways to reduce particle emissions from

small-scale biomass combustion systems are primarily the utilization of modern combustion

technologies and secondarily the use of small-scale precipitation devices as it is estimated

that it might be challenging to reach future ambitious emission limits even for new

combustion systems. At present, there are several small-scale precipitation devices

(primarily electrostatic precipitators) under development and some are already available on

the market [ 2].

1.1 Target group

The aim of this work is to present generic guidelines for the design and application of

electrostatic precipitators for residential biomass combustion systems. Criteria to be fulfilled

by a residential ESP that works well are provided. The target groups of this report are

manufacturers of residential combustion devices, retailers, and central authorities.

Furthermore, the report may also be of interest to consumers and researchers.

2 Definitions and limitations

2.1 Basic definitions

Electrostatic precipitators (ESPs) have been used for decades to clean particles from flue

gases in large-scale industrial processes. To clean such relatively large gas volumes, the

most suitable ESP design is usually of the parallel-plate type, as outlined to the left in

Figure 1. On the other hand, for the smaller gas flow of a residential furnace, a tubular ESP

design (to the right in Figure 1) may be preferred because it is easier to accommodate either

in or on top of the chimney.

The main (vital) parts of an ESP are:

• Discharge electrode (which confers a charge on the particles entering the ESP)

• Collecting electrode (to which the charged particles are attracted)

• High-voltage power supply

Necessary auxiliary parts include high-voltage insulators, a control system and a system to

remove dust collected on the collecting surfaces.


2 Guidelines for the design and application of particle precipitators for residential biomass combustion

Figure 1: Principles of two common ESP designs: a) Parallel plates, b) Tubular.

An ESP for residential furnaces may be placed somewhere between the furnace and the exit

of the chimney. For example, if the house has a boiler, the ESP can be placed in the boiler

room. For a chimney stove, the ESP may be placed either next to the stove, in the flue gas

duct, or outdoors on the top of the chimney.

2.2 Limitations

The present guidelines cover electrostatic precipitators for residential biomass combustion

systems with a nominal boiler capacity up to 50 kW as well as for residential wood stoves.

Furthermore, these guidelines are limited to generic descriptions of what to expect from a

residential ESP. How this can be accomplished is not described in detail, although a few

examples are given in certain instances in order to improve understanding of the issue.

3 Design criteria

Design criteria can be derived from the basic technological functions of ESP systems as well

as from a practical user’s perspective, i.e. the ESP should be easy to use. In these

guidelines, both aspects are covered together.

3.1 Technology

Electrostatic precipitators seem to be the most promising option for small-scale residential

applications. Other options such as as catalytic converters and ceramic filters may exhibit a

high pressure drop. Catalytic converters are typically not available during start-up (where

high emissions occur) and may show problems regarding deactivation and deposit formation.


Guidelines for the design and application of particle precipitators for residential biomass combustion 3

3.2 Efficiency

An ESP should aim to considerably reduce particle emissions and meet emission

requirements (for example 40 mg/mN3, at 13 % O2, for wood stoves in Germany after

01/01/2015 [ 3]). To meet such requirements, a residential ESP should typically show a

collecting efficiency of at least 75% during normal operation. In large-scale ESP systems, the

collecting efficiencies are usually higher than 90% and that should also be the target for

small-scale filters.

There are many design parameters of the ESP as well as process parameters of the

combustor that determine the actual collecting efficiency. Basically, under ideal conditions

(turbulent flow, perfect mixing and immediate charging etc.) the collecting efficiency (η) may

be estimated from the Deutsch equation [ 4]:

Deeη −

−= 1

in which De is the Deutsch number:

Q

ADe ω=

where ω is the migration velocity of particles, A the surface area of collecting electrodes and

Q the volumetric gas flow.

The Deutsch equation shows that the collecting efficiency increases with the ratio between

collecting surface area and gas flow. For instance, this ratio can be improved by using longer

electrodes in a tubular ESP, although a larger surface area might also increase the power

required to operate the ESP. Another way to expand the collecting surface area is by

enlarging the diameter of a tubular ESP. In this case, the longer distance between electrodes

calls for an increased voltage between electrodes to attain the same electrical field strength.

Another consequence is that the particles in the flue gas will have to migrate a longer

distance, on average, perpendicular to the direction of the flow to reach the collecting

electrode. Thus, in practice, the ESP performance will deteriorate if the distance between

electrodes becomes too wide. The Deutsch equation also shows that the collecting efficiency

is improved by lowered flue gas temperature (which affects Q). The temperature also affects

the resistivity of the fly ash, which is one of many process parameters that determine the

value of the migration velocity (ω) in the Deutsch equation. This ω is a key design parameter

for ESPs and it depends on a large number of factors such as: particle size distribution,

particle density, gas velocity, gas composition, gas temperature, etc. It is also affected by the

voltage between electrodes since the force acting on charged particles depends on the

electrical field strength. A deeper and more detailed explanation, as well as calculation of the

migration velocity, can be found in [ 5]

It is important to be aware of the fact that for low resistivity dust, such as fly ash from wood,

the collection efficiency of an ESP increases with the power consumption, which can be



controlled by the high-voltage supply unit. This trend holds up until a threshold voltage is

reached where sparkovers or even arcs start to form between electrodes. At the onset of

sparks, a further increase of power does not improve ESP performance, rather on the

contrary. For maximum collecting efficiency, the voltage between electrodes should be as

close to the onset of sparks as possible. However, in some cases it may be possible to

significantly lower the power consumption while maintaining sufficiently high collecting

efficiency. Nevertheless, when comparing ESP performances it is important to compare both

collecting efficiency and power consumption.

The collecting efficiency of an ESP will generally benefit from clean electrode surfaces,

because dust build-up in the filter may cause entrainment of dust into the gas stream and/or

lower the sparkover onset voltage. Both these processes reduce the collection efficiency.

Therefore, it should be a goal of an ESP-designer to keep surfaces clean during long term

operation, for instance by including some kind of cleaning mechanism in the system.

3.3 Installation

Correct installation is crucial for the function of an ESP. Installation should be performed by a

professional. To ensure proper installation, a well-written manual should be available. For

chimney-top devices, lightning protection shall be considered.

3.4 Pressure drop

A design with a low pressure drop over the ESP is required in order to avoid influences on

the boiler/stove operation. Otherwise, a high pressure drop may cause poor combustion

conditions and decreased burnout quality due to reduced combustion air supply which

typically results in increased CO, OGC and PM emissions. The low pressure drop is

particularly important for stoves and other combustion systems that depend on natural draft

to supply the air.

3.5 High voltage and electrode

A residential ESP needs a high-voltage power supply, typically a voltage in the range of 15-

30 kV. Furthermore, the recommendations with respect to high voltage and the discharge

electrode are:

• Thin discharge electrodes with sharp surfaces are preferred to achieve a high

charging efficiency (e.g.: needle or sawtooth profile, etc.)

• Frequent sparkovers should be avoided by the high-voltage control system (since

they are noisy, reduce the collection efficiency and increase the power consumption).

Some observed reasons for frequent sparkovers are:

o Too flexible discharge electrode, which can start to vibrate during operation



o Poorly aligned electrodes, with locally too short distance between electrodes

o Conductive deposits on insulator parts

o A high moisture content in the flue gas and an ESP temperature below the

dew point causes condensation

o Deposition of particles as well as surface condensation of condensable

organic vapours on the insulator (should be avoided, by e.g.: purge air,

heating or an appropriate geometric design)

3.6 Control system

A control system is important for the function of an ESP.

For automated boilers and stoves, an interface between the ESP and the control system of

the stove/boiler should be provided to appropriately manage start-up and shut down

procedures. For stove and boiler systems without automatic control, the ESP control system

should detect start-up and shut down of the furnace by using a temperature sensor in the flue

gas duct, e.g. by temperature changes in the flue gas.

The control system should also include the operation of a cleaning system for the electrode

and the collection surfaces. It should also be possible to manually suppress the cleaning

process, e.g. in case of chimney sweeper’s measurements.

3.7 Maintenance and availability

A basic requirement is that an ESP should be available for a whole heating season without

any maintenance, except for cleaning by a chimney sweeper, if required. A robust design is

necessary to avoid damages during the sweeping process. Simple, safe and quick access

should be provided for cleaning and inspection. Weak points may be the insulators and the

high voltage system. Robust and well-proven technical solutions are a basic requirement in

order to ensure reasonable availability.

There is a need to consider how the efficiency and the availability are influenced by

condensable and sticky particles present in the flue gas as a result from poor combustion

conditions, which are typical for old stoves/boilers as well as during start-up. For instance, an

appropriate automatic cleaning system may prevent deteriorated performance from

particulate fouling.

It is useful to have internal logging of the ESP operation versus operation of the combustion

device in order to determine the unit’s availability and to be able to prove the operation to any

authority, if such information is collected during inspection routines.



3.8 Applicability

There should be a special focus on the applicability of filters for old systems which exhibit

great particle reduction potentials and rough operation conditions (high amounts of

condensable organic compounds and soot). Moreover, an ESP precipitator can also be

suitable for stoves, although in this case only roof-top or in-chimney applications make

sense.

Suppliers should be asked for the applicability of their ESP for different combustion devices

and be able to provide references that the ESP can fulfill required collecting efficiency and

reasonable availability.

3.9 Cleaning

The dust collected on the surfaces in the ESP has to be removed sooner or later. There are

two main methods to accomplish this:

• The ESP is equipped with an efficient, automatic cleaning system (at least for the

collecting surfaces but it is also recommended to clean the high voltage

electrodes). This is typically done by either: vibration, a brush, or a water spray.

• Cleaning by the chimney sweeper or the user.

Automatic cleaning systems for the electrode and the collection surfaces are strongly

recommended.

3.9.1 Solid waste: ash, coke and soot

Irrespective of the cleaning method there will be a solid waste product consisting of ash,

coke and soot. This solid waste may contain harmful substances and should be handled as

hazardous material. It has to be handled according to the legislation in the respective

country. Direct contact and inhalation of the particles should be avoided.

3.9.2 Waste water

If waste water is produced (e.g. by using a water spray cleaning system) it has to be handled

according to the legislation in the respective country.

3.10 Safety

For safety reasons the ESP has to be a closed system to prevent leakage of flue gas to the

surroundings, unless it is outdoors on the top of a chimney. Other safety aspects that should



be considered are risks connected to the use of high voltages. Because of this, grounding of

the ESP in a correct manner is important. As mentioned, chimney-top devices need to be

provided with lightning protection. A minimum requirement is a potential equalization.

High temperature surfaces should be avoided to prevent burn injuries.

Fire safety shall be guaranteed during soot fire in the chimney, see for example the German

safety test program [ 6]. The regulations may differ between countries and may also be less

strict for chimney-top installations [ 7].

Safety instructions should be available on or close to the filter where maintenance and

cleaning procedures are performed and ports should be labeled to prevent opening during

operation.

3.11 Noise

Noise is a basic design criterion which should be considered. The origins of noise from an

ESP could be for example from sparkovers, cleaning equipment and ventilation. However,

there is no known information about acceptable noise levels available today. A certain level

of noise should be acceptable if the ESP is on top of the chimney or placed in the boiler

room. More work is needed to define acceptable noise levels. As already mentioned frequent

sparkovers should be avoided by the high-voltage control system since they are noisy and

also reduce the collecting efficiency.

3.12 Weather conditions

In many cases, residential ESP units are located on top of the roof or directly in the chimney.

In such cases it is important to ensure that the ESP withstands different weather conditions,

for example rain, snow and storms.

3.13 Costs

The costs of a residential ESP should be reasonable with respect to the costs of a stove or a

residential boiler. Thus, rather simple and robust solutions are required for residential

applications.

Furthermore, the operational cost of an ESP unit should be reasonable. Operation costs are

directly proportional to power consumption if the pressure drop of the ESP system is small.



4 Quality assurance

The function of an ESP should be verified by an independent testing laboratory (especially its

efficiency and availability). However, no common standard test method is currently available

for type testing of residential ESPs. Therefore, it is difficult to compare test results from

different laboratories. Below, some recommendations concerning the evaluation of the

performance of residential ESPs are given.

4.1 Testing methods

General aspects regarding measurements at ESP units:

• The influence of charged particles on particle losses in sampling lines is not fully

understood yet. More investigations are needed to provide recommendations with

respect to this.

• The collecting efficiency of filters located on chimney tops cannot easily be evaluated

in the field. Thus, appropriate test-stand measurements are needed. Furthermore,

their availability must be monitored – e.g. by logging the voltage of the system

together with the flue gas temperature as an indicator of the furnace operation.

• At test stand measurements, the field operation conditions of a specific filter have to

be considered:

o The position of the filter (directly coupled to the stove/boiler versus in-chimney

or roof-top) should be comparable to the foreseen installation, because the

position has a strong influence on flue gas temperature and aerosol formation

from organic compounds.

o The temperature at the filter inlet should be comparable to field conditions.

o Temperature losses over the filter should be considered in the evaluation due

to possible formation of organic aerosols.

o Simultaneous dust measurements before and after the filter are

recommended.

• As a recommendation, the test report shall provide the following information:

o Mean collecting efficiency

o Minimum collecting efficiency (at a confidence level of 95%, calculated from a

number of replicate tests performed)

o Average PM-concentration in the clean flue gas

o Description of combustion system used

o Fuel specification

• Test measurements should preferably be performed after the filter has been in

operation for a considerable time, because the collecting efficiency is usually

unrepresentatively high when the unit is new and clean.

Considering these aspects two different evaluation criteria for combustion systems equipped

with an ESP can be defined:



1. The mean and minimum collecting efficiencies of the ESP calculated from the PM

concentrations in the flue gas at ESP inlet and ESP outlet. This criterion is of interest

from a technical viewpoint.

2. Emissions of the system calculated from the PM emission in the flue gas at ESP

outlet. By measuring in diluted flue gas (diluted to below 50°C), the measured particle

concentration will include condensable organic compounds that still are in gas phase

at the ESP outlet. This measurement technique provides a more relevant value from

an environmental viewpoint.

For combustion systems operating at high efficiencies (low flue gas temperatures) and good

burnout (low amounts of condensable organic compounds), no significant difference is

expected between measurements performed in diluted and undiluted flue gas. On the other

hand, for old systems (poor burnout, high flue gas temperatures) significant differences may

occur. For old systems, lowering the operating temperature in the ESP improves the

precipitation of particles. Moreover, lower operating temperatures typically reduce the

stickiness of the particles and enhance the formation of particles from gaseous organic

species upstream the ESP, which improve the precipitation potential. Lower operation

temperatures can be achieved by chimney top applications.

A standard test method for determining the collecting efficiency of a particle precipitator is

currently being elaborated in Germany. This VDI Guideline 33999 [ 8] will have the status of a

full national standard. It is supposed to be suitable for secondary retrofit units applying

electrostatic, filtering, catalytic, condensing, washing or centrifugal principles, or a

combination of these.

5 Conclusions

Electrostatic precipitation (ESP) of fly ash is a well-established technique for flue gas

cleaning in industrial processes. Smaller scale versions, to suit residential furnaces, are

under development. In order to become widely used, such ESPs have to meet some criteria

regarding efficiency, cost and availability. Furthermore, aspects of safety, noise and

convenient installation have to be considered.

There is a lack of commonly accepted methods for testing the efficiency of residential ESPs.

The set-up and sampling methods used may considerably affect the test results. Thus,

caution should be applied when comparing results from ESPs tested under different

conditions.



6 Acknowledgement

This project was funded within the ERA-NET Bioenergy programme “Clean Biomass

Combustion”. The project consortium gratefully acknowledges the financial support for

carrying out the project provided by:

Austrian Research Promotion Agency (FFG), Austria

Tekes – the Finnish Funding Agency for Technology and Innovation

Bayerisches Staatsministerium für Ernährung, Landwirtschaft und Forsten (BayStMELF)

Swedish Energy Agency, Sweden

NCBiR – National Centre for Research and Development, Poland

Sustainable Energy Authority of Ireland, Ireland

7 Related literature

1. Air Quality EU Directive 2008/50/EG

2. OBERNBERGER I., MANDL C., Survey on the present state of particle precipitation devices for residential biomass combustion with a nominal capacity up to 50 kW in IEA Bioenergy Task32 member countries. Final report of the related IEA Task32 project, BIOS BIOENERGIESYSTEME GmbH (Ed.), Graz, Austria, 2011

3. Novelle 1. Bundesimmisionsschutzverordnung (BImSchV), Germany, 26.01.2010

4. Deutsch W., Ann. Phys (Leipzig) 68, 335-344, 1922

5. Lloyd D., Electrostatic precipitator handbook, IOP Publishing Ltd Bristol, England, 1988

6. DIBT (Deutsches Institut für Bautechnik): Vorläufiges Prüfprogramm für Staubabscheider für handbeschickte Kleinfeuerungsanlagen, 2008

7. Vereinigung Kantonaler Feuerversicherungen (VKF): Brandschutzmerkblatt - Einbau von Feinstaubabscheidesystemen (2011). www.vkf.ch

8. VDI Guideline 33999 (Working Title: Emissionsminderung — Kleine und mittlere Feuerungsanlagen - Prüfverfahren zur Ermittlung der Wirksamkeit von nachgeschalteten Staubminderungseinrichtungen

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